The present invention relates in general to triggered oscillators and in particular to a triggered oscillator having a frequency locked output signal.
Sampling oscilloscopes were developed more than twenty years ago to observe small, fast-changing signals to which conventional oscilloscopes could not respond due to limited bandwidth or risetime characteristics. Sampling is a now well-known technique wherein a signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of an electrical signal during that period. Each sample taken in this manner is processed by electronic circuits and displayed as a dot on a cathode-ray tube (CRT) screen at an appropriate position corresponding to the relative timing and magnitude of the sample. Since the samples appear on the CRT display as dots, a large number of samples are required to accurately reconstruct a waveform. Generally speaking, sampling is most practical when the electrical signal is repetitive in nature since it is impossible to acquire all of the needed samples during a single cycle of all but relatively low frequency signals. Indeed, one of the advantages of sampling is that at least one-sample can be acquired from each of a large number of cycles of a high frequency signal, and a representative waveform may be reconstructed and displayed therefrom.
High frequency noise in a waveform can cause a sampling system to distort the waveform display, particularly if a sample happens to be taken at a noise peak. One method of reducing the effects of noise would be to sample a periodic waveform repeatedly at similar times with respect to an event (such as a zero crossing) occurring in repetitive sections of the waveform and then to average the digitized results to determine the actual magnitude of the waveform at each sample time. For instance if 1000 repetitive waveform sections were sampled at similar points, and the sample values were averaged, the effects of noise in any one sample would be reduced by a factor of 1000.
In sequential sampling systems waveforms are sampled at periodic intervals. In order for a sequential sampling system to be used in conjunction with such an averaging method for reducing noise effects, the sampling frequency would have to remain constant and the sample timing with respect to the repetitive event in a sampled waveform would have to remain constant during several repetitive sections of a waveform. However, in sequential sampling systems of the prior art, the point at which sampling begins following a triggering event in the waveform cannot be precisely controlled. Since sample timing is typically controlled by a strobe signal generator which initiates sampling in response to a periodic input signal produced by an oscillator, what is needed is a triggered oscillator for producing a periodic clock signal of precisely controllable frequency in which the first cycle of the periodic signal coincides with a triggering signal derived from a repetitive triggering event in a waveform.
A triggered oscillator of the prior art includes a NOR gate having an output delayed by a delay circuit and then fed back to an input of the NOR gate. An active low trigger signal is applied to a second input of the NOR gate. When the trigger signal is asserted, the output of the NOR gate oscillates with a frequency determined by the delay time associated with the delay circuit, but when the trigger signal is not asserted the NOR gate output does not oscillate. However due to temperature changes, differences in components utilized in the oscillator and other sources of error, the frequency of the triggered oscillator output signal is not accurately predictable and tends to change over time.